In nuclear magnetic resonance, the excitation of transverse magnetization over very large bandwidths represents a difficult challenge, particularly if it is essential to the success of the experiment that the phase of the magnetization be independent of offset. (Offset is the difference between the carrier frequency of the excitation rf pulse and the frequency of the excited spins.) Excitation with a simple radio-frequency pulse gives rise to a significant phase-dispersion if the offset exceeds the amplitude of the rf field. If the signals are recorded immediately after excitation, it is possible to compensate the resulting frequency dependence of the phase across the spectrum by standard phase-correction procedures. However, if the initial excitation is followed by spin-locking or various coherence transfer sequences, the initial phase-dispersion of the magnetization may lead to deleterious effects.
With the advent of "composite pulses", it has become possible to combat the effects of phase-dispersion very effectively (see M. H. Levitt, Progr. NMR Sectrose. 18 (1986) 61, which is hereby incorporated by reference herein). Composite pulses are comprised of sequences of closely-spaced radio-frequency pulses with different phases and durations, but where the rf amplitude and carrier frequency are usually constant. Generally speaking, the rf amplitudes used in composite pulses tend to be high, so that powerful amplifiers are required, and the effects on the sample are not always harmless, particularly for biological systems or samples at very low temperatures. The invention provides an alternative approach which requires much lower rf amplitudes.
In a variety of magnetic resonance experiments, it has been demonstrated that it can be advantageous to replace non-selective pulses by frequency-modulated "chirp" pulses (see J.-M. Bohlen, M. Rey and G. Bodenhausen, J. Magn. Reson. 84 (1989) 191, I. Burghardt, J.-M. Bohlen and G. Bodenhausen, J. Chem. Phys. 93 (1990) 7687, and G. Bodenhausen, J.-M. Bohlen and M. Rey, U.S. Pat. No. 5,126,671, Jun. 30, 1992, which are hereby incorporated by reference herein). The rf frequency of a chirp pulse is swept from a lower rf frequency to a higher rf frequency or vice versa within a predetermined time. Chirp pulses allow one to excite and refocus magnetization over very large bandwidths using limited radio-frequency amplitudes. While they have numerous advantages, sequences of chirp pulses also have their "Achilles' heel" since they tend to have a long duration. As a result, the experiments tend to be prone to signal losses due to transverse relaxation.
It is an object of the invention to avoid or reduce such signal losses.